The present invention relates to a structural material of the sandwich-type construction. Materials of this type are used primarily to achieve a high strengh-to-weight ratio, and they are noted for their stiffness and crush-resistance. Typically, lightweight, bulky, relatively weak materials such as plastic foams are sandwiched between relatively heavy and high strength facing sheets (or simply "facings") such as fiber-reinforced plastic. The facings are bonded to the core, resulting in the "sandwich" characterization.
There are many products which use sandwich panel structures of the type with which the present invention is concerned, such as boat hulls, camper tops, storage tanks, and many others. Further, there are many manufacturing techniques employed in joining the facings to the core. One such technique is herein referred to as "gravity" or "contact" molding. In this manufacturing method, a female or male mold section is waxed, covered with a gel coat (or other finish material), and then a layer of fiberglass reinforcement and polyester resin is applied. Before the resin has cured, a core material is placed in the mold, and a second fiber-reinforced plastic layer is applied to the top of the core material. The composite assembly is permitted to rest until the plastic has cured, during which time the core material becomes bonded to the surface sheets.
In this method of assembly, it is desirable that the core material be flexible enough to conform to the contour of the mold without the application of substantial external pressure (which might require special holding jigs) since this would also require an elaborate mating mold section. In the case of a boat hull, for example, such curvature may be relatively small in certain areas, and it also may be complex--that is, two different degrees of curvature may be required.
Although achieving curvature of the core material in a pressure molding method employing complementary mold sections is not difficult, this has presented a problem in the case of gravity or contact molding of sandwich panel constructions, and there have been attempts to provide flexible core materials in such structures. In gravity molding a slight pressure (such as the weight of an object) might be used to hold the materials in contact until the resin cures.
Some prior attempts to provide flexible cores are shown in U.S. Pat. Nos. 3,139,371; 3,573,144; and 2,036,467. For the most part, such prior attempts require the use of individual block members (of balsa wood, for example) together in one fashion or another, such as with a scrim, to yield flexibility. Such attempts have proved to be expensive and have presented problems in attaching the scrim material to the faces of the blocks. If not properly applied, the scrim may also present a problem in acting as a parting surface for the outer facings applied later.
According to the present invention, the core is made of a plurality of blocks of lightweight core material arranged side-by-side or in strips. The blocks may be of rigid plastic foam (such as polyurethane), balsa wood, or they may themselves be composite structures such as a block of plastic foam laminated to a strip of end grain lumber. The lumber provides additional crush resistance when used with softer rigid foam plastic. The blocks or strips are joined by corrugated sheet material, such as double-sided corrugated paperboard commonly used to make cartons. The flutes of the corrugations may run parallel to the direction of extension of the core strips, or they may run transverse of the direction of extension of the core strips. In either case, it has been found that the resulting core material is flexible to the extent that it will conform to mold shape with little or no external pressure. If greater flexibility is desired, the corrugated sheet material may be crushed either prior or subsequent to its incorporation into the core material or the core material may be scored with a saw to permit the strips to flex.
The present invention thus provides an economical core material for a sandwich panel construction which is flexible enough to conform to a mold in a gravity molding method, but which has the crush resistance and high strength-to-weight ratio of prior constructions. This is achieved because the corrugated material provides a plurality of line connections between the core strips (along the ridges of the corrugations) with flexible material between the line connections. Thus, the corrugated material acts as a hinge between adjacent core strips. The degree of hinge action is increased by using crushed corrugated material, but the degree of flexibility does not appear to depend heavily on the direction in which the flutes of the corrugated material extend. Further, this "hinging" action is two-dimensional--that is, looking from an edge of the core material transverse of the direction of extension of the strips, a forward strip can be rotated counterclockwise, and a rear strip clockwise, and flexibility is apparent.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.